The transparency of glass or plastic, in the form of doors, windows, lenses, filters, display devices (e.g., display panels) of electronic equipment, and the like, can be impaired by glare or reflection of light. To reduce the amount of glare, for example, on plastic or glass, the surface typically includes a single layer of a metal oxide (such as silicon dioxide), a metal fluoride, a metal nitride, a metal sulfide, or the like. Such coatings function as antireflective coatings.
Glass surfaces, for example, have about 4% surface reflection. With the aid of specialized coatings, such as metal oxides, this surface reflection can be reduced to less than about 0.5% average integrated intensity in the visible region of the spectrum at 450-650 nanometers (nm). The coatings can be multilayers of dielectric materials deposited in submicrometer thicknesses arranged to cause constructive or destructive interference of light waves of different wavelength. Antireflective materials in the visible region typically consist of three or four layers, two of which are of different materials, of alternating high and low index materials. Layers of quarter-wavelength or half-wavelength in optical thickness are typically used in the design of such materials.
Antireflective (AR) film stacks prepared by vacuum deposition (e.g., vacuum sputtering) of metal oxide thin films on substrates made of plastic, particularly flexible plastic, or glass, are particularly useful in display devices of electronic equipment. Such metal oxide films are relatively porous and consist of clusters of particles forming a relatively rough profile, which helps reduce glare and reflection. When such materials are conductive, they also help reduce static discharge and electromagnetic emissions. Thus, the primary application for these coatings is to provide contrast enhancement and antireflective properties to improve the readability of display devices, such as computer monitors.
Vacuum deposited (e.g., sputtered) metal oxide antireflective coatings are generally durable and uniform. Also, their optical properties are controllable, which makes them very desirable. They also have very high surface energies and refractive indices, however. The high surface energy of a vacuum deposited (e.g., sputtered) metal oxide surface makes it prone to contamination by organic impurities (from sources such as fingerprints). The presence of surface contaminants results in a major degradation of antireflectivity properties of the metal oxide coatings. Furthermore, because of the high refractive indices, surface contamination becomes extremely noticeable to the end-user.
Unfortunately, the high surface energy makes a vacuum deposited (e.g., sputtered) metal oxide surface difficult to clean without the use of environmentally undesirable solvent-based cleaners. Furthermore, removal of the surface contaminants can detrimentally affect the antireflective properties of the surface if the cleaning process leaves residue behind. Thus, a need exists for a protective coating on an antireflective surface that is relatively durable, and more resistant to contamination and easier to clean than the antireflective surface itself.
Numerous attempts have been made to provide antisoiling characteristics to an antireflective surface. This has been accomplished by providing antisoiling characteristics to the antireflective coating itself, or by providing an antisoiling coating over the antireflective coating. Examples of such antisoiling overcoatings are described in Applicants' Assignee's copending patent application U.S. Ser. No. 08/902,666, filed Jul. 30, 1997 (Pellerite et al.), and in JP Document 9-127307 (Sony Corp.) and U.S. Pat. No. 5,622,784 (Okaue et al.). The materials disclosed in the former document, however, are not generally appropriate for continuous coating techniques. Materials disclosed in the latter two documents, which are within the general type of compounds used in Comparative Examples B, I, O, and P herein, do not provide sufficiently durable antisoiling coatings. Although perfluoroether derivatives, such as that commercially available under the trade designation KRYTOX 157 FS(L) from E.I. DuPont de Nemours Co., Wilmington, Del., have been used as lubricants on surfaces of magnetic media articles and hard discs, they, alone, provide little antisoiling characteristics when applied to a transparent substrate as shown in Comparative Example D herein. Thus, a need still exists for materials that form durable antisoiling coatings suitable for application to substrates, particularly flexible substrates, in continuous coating techniques.